Immunoglobulin (IgG and IgA) and complement (C3) concentrations in uterine secretion following an intrauterine challenge of Streptococcus zooepidemicus in mares susceptible to versus resistant to chronic uterine infection

The validity of measuring concentrations of immunoglobulins in undiluted uterine secretions was established. The concentrations of IgG, IgA, and cleavage factor C3 of the complement system in uterine secretions were compared in mares with different resistance to chronic uterine infection (CUI). The uteri of mares susceptible (n = 6) and resistant to CUI (n = 5) were inoculated with 5 x 10⁶ Streptococcus zooepidemicus when the mares were in estrus. Uterine secretions were sampled, and sampling was immediately followed by a uterine lavage at 5 and 24 h after bacterial inoculation. During a subsequent estrus, bacterial inoculation of the uterus was repeated, and samples were taken from the mares 12 and 36 h later. The fraction of uterine secretion in each uterine washing was determined by dividing the protein concentration of the undiluted uterine secretion by the protein concentration of the corresponding uterine washing. There was a significant correlation between measured concentrations of immunoglobulins in uterine secretions and calculated concentrations in the uterine washings (p < 0.05). Concentrations of IgG and C3 in uterine secretions declined similarly in both susceptible and resistant mares during the first 24 h after bacterial inoculation (p < 0.04). In contrast to the susceptible group, which showed a continuous decline at 36 h, resistant mares demonstrated an increased concentration of both IgG and C3 at this time. Concentrations of IgA did not differ between susceptible and resistant mares. It was concluded that the described method of sampling uterine secretions was useful for analyzing IgG and IgA. Since this study was unable to detect differences in IgA, IgG, and C3 concentrations during the first 24 h of infection, the impaired uterine defense in mares susceptible to CUI is not likely to be due to local immunoglobulin deficiency.